Poor neuromuscular control, such as delayed muscle recruitment or alterations in levels of muscle activation, can lead to abnormal loads on joints and ligaments of the spine, leading to spinal instability and lower back pain (LBP). Active contraction of trunk muscles helps to control inter-segmental movement and stability in the lumbar spine by adding stiffness to the trunk. Muscle dysfunction associated with LBP has been thought to involve deep, local trunk muscles - the transversus abdominis (TA) and multifidus muscles. These muscles are hypothesized to provide stability to the lumbar spine. Treatments which focus on retraining TA and multifidus by using low level isometric contractions of these specific deep trunk muscles during particular tasks and functional activities have been effective at reducing LBP and improving function for selected subgroups of LBP subjects. The precise muscle dysfunction associated with LBP has not been satisfactorily characterized, in part because muscles such as TA and multifidus are difficult to characterize directly given their anatomical location. However, analytical modeling can assist in characterizing these deeper muscles. A more complete characterization of the function of these muscles can lead to more precise treatments of LBP. [unreadable] [unreadable] The three major aims of this project are: 1) to characterize the pre-treatment motor control alterations (alterations in muscle latencies or in modulation of muscle activity level) in subjects with spondylolisthesis and mechanical LBP compared to healthy control subjects, and to determine whether specific deep trunk-muscle exercises (low level isometric contractions of TA and multifidus) improve these alterations compared to a general exercise protocol. We hypothesize that prior to treatment, muscle latencies will be earlier in LBP groups compared to the control group and that the latencies will improve (increase) in LBP subjects receiving specific trunk exercise protocol but not in subjects receiving a general exercise protocol at all time points post-treatment; 2) to employ biomechanical modeling to study mechanisms underlying these specific deep trunk muscle exercises. We hypothesize that increased activation of TA and multifidus muscles increases stability of the trunk by reducing intervetebral motion; and 3) to determine the effects of specific trunk exercises compared to more general strengthening and endurance exercises on pain and function during activities of daily living in the LBP subjects. We hypothesize that subjects in a specific exercise protocol will have decreased pain and increased functional ability compared to subjects in a general exercise protocol, at all time points post-treatment.